This invention relates to aqueous printing inks and, more particularly, to aqueous, pigmented, red ink jet ink compositions which meet the stringent postal requirements and are thus suitable for use in printing postal indicia.
Ink jet ink printing is a form of digital printing in which droplets of ink are made to be ejected (i.e., "fire") from an orifice in a printhead or a spray nozzle in response to an electronic signal from, for example, a computer. Examples of ink jet printing include thermal ink jet printing, piezoelectric ink jet printing, continuous ink jet printing and air brush printing.
In the past decade, ink jet printing, particularly the thermal and piezo forms, has become extremely popular for home office, small office and personal printer applications, primarily due to its relatively low cost, speed and quite operation. In more recent years, the popularity of ink jet printing has increased even further due to the introduction of systems (printers, software, media and inks) offering very high quality (near photographic) color and graphics capabilities.
Postal authorities around the world require specialized indicia be used when the postage is applied to the mail by machine ("metered mail"), as opposed to when stamps are used for postage. Typically this indicia identifies the amount of the postage, the location of origin of the mail and often will identify the registered user of the device used to print the indicia. Because the machines that apply the postal indicia are, in effect, printing money, they are typically very strictly monitored. In addition, to avoid fraud, survive the elements, and to facilitate automated mail handling, the indicia must meet strict requirements. For example, the indicia must be readable after being immersed in water for 24 hours. It must be resistant to bleaching or removal with other solvents to prevent tampering with the postal amount. In the US and in some other countries, it must also be fluorescent on a variety of envelopes so that it is readable by a facer-canceller. It must be smudge and smear resistant to remain readable with repeated handling.
For many years, the postal indicia was applied with a stand alone metering device. The device was, in effect, a relatively sophisticated stamping device in which the postal amount was set by the user and the envelope was passed through the device to be metered and the appropriate postage applied. The ink used for the indicia was a fluorescent red ink applied with a stamp. Recently, ink jet based products have been introduced on the market for applying postal indicia to envelopes.
Despite its commercial success and Post Office approval, the inks used in the commercially available ink jet postal metering devices are inferior to the stamping inks previously used for postal metering in the sense that they do not satisfy some of the postal criterion mentioned above. For example, while the inks used in these commercial devices satisfy the fluorescence requirements, they do not perform well in the water immersion test or the bleach resistance test. Accordingly, there is a need in the art for aqueous red ink jet inks that can satisfy the demanding postal criterion for use in metering mail.